The LTE-A Standard (Long Term Evolution-Advanced) enables a mobile telephone to operate in a so-called carrier aggregation mode (=CA Mode) in order to utilize a higher bandwidth in the downlink and/or in the uplink and thus to achieve a faster uplink or downlink speed. The Carrier Aggregation Mode is defined both for FDD (Frequency Division Duplexing) and for TDD (Time Division Duplexing) bands and enables two bands to be utilized simultaneously, for example, in downlink mode (receiving mode) for a conversational or data connection. According to Standard 3 GPP TS 36.101, a series of so-called interband carrier aggregation pairs are already defined by 3GPP. A list of the band combinations currently discussed and partially already examined is given in FIG. 15. For example, according to a proposed CA combination, a mobile telephone can receive in band 20 and additionally receive in band 7 (RX-CA). Similarly, it is possible that transmission occurs in bands 20 and 7 and reception occurs either in band 7 or band 20 (RX-CA). The defined carrier aggregation band pairs are then related to bands, to the application for Rx or Tx mode and to the duplex mode, i.e., to FDD and TDD.
To meet the requirements for the CA mode, the corresponding duplexers should be able to be active at the same time in the front-end circuit of the mobile telephone without interfering with one another. If, however, two duplexers are connected to the same antenna terminal, this, as a rule, represents a quadplexer which must be optimized for this special application as a new component.
Overall, it is technically simple to implement those of the proposed band pairs of which the bands are far enough apart in frequency, for example, bands 20 and 3. These can be combined at a common antenna terminal with a simple duplexer which exhibits good isolation of, typically more than 20 dB. In this manner, it is possible to ensure that the duplexers connected to the common antenna terminal do not interfere with one another so that a signal is conducted exclusively via the required duplexer and does not generate any unnecessary losses in the other duplexer.
In addition to the low loss, a quadplexer must have good isolation between TX and RX subbands, that is to say between the corresponding transmitting and receiving bands. This applies both to the TX/RX isolation within the same band and between the TX mode and of the first band and the RX mode of the combined second band. These requirements are already met for the cases in which a diplexer can be used.
In the cases of proposed CA band pairs, however, in which the frequencies of the bands are close to one another, for example, in the cases of the CA band combinations 5 & 17, 8 & 20 or 2 & 4, conventional diplexers cannot be combined simply at the antenna terminal. In these cases, the isolation between low-pass and high-pass of the diplexer is not adequate for mutual isolation of the bands and the duplexers must be directly matched to one another. For this purpose, in the passband of one duplexer the impedance must appear to be infinite at the antenna terminal of the other duplexer, which usually requires a corresponding phase rotation of the impedance. The duplexer, therefore, must have a high reflection coefficient at the antenna terminal for frequencies of the other band, that is to say outside its passband. This can be achieved by the impedance being rotated towards infinity with the aid of a phase shifter in the same frequency band.
FIG. 1 shows an arrangement, known per se, of a first and second duplexer DPX1, DPX2 which are connected to a common antenna terminal AT. Between antenna terminal AT and the input of each duplexer, a phase shift circuit PS1, PS2 is arranged in each case which is intended to rotate the impedance in the passband of the other duplexer in each case towards infinity.
Typically, the TX filter (transmit filter) is designed as a reactance filter with ladder-type arrangement in the duplexers which is constructed from both serially and parallel interconnected resonators. The resonators can be constructed as SAW or BAW resonators. The respective RX filter (receive filter) can also have other filter components apart from the ladder-type structure, for example, acoustically coupled resonator structures such as, for example, DMS structures.
The transfer characteristic of a ladder-type filter has three different characteristic sections: the out-of-band suppression, the depth of the poles or attenuation peaks on both sides of the passband and the passband itself. The individual duplexer itself uses advantageously characteristic deep poles (notches) in the transfer characteristic in order to achieve a very high attenuation between the TX section and the RX section. While the frequency of the pole below the passband is determined, as a rule, by the resonant frequency of the parallel resonators, the frequency of the pole above the passband is determined by the antiresonant frequency of the parallel resonators and the resonant frequency of the series resonators. In addition, the series resonance of the series resonators must be within the passband, naturally, as does the antiresonance of the parallel resonators.
To optimize the precise position of attenuation peaks relative to the passband or to increase the bandwidth of the resonators, it is known to connect inductances in series with the parallel resonators. This makes it possible to create further poles in the filter response. Since the additional inductances, however, have a limited quality factor (Q factor), an interconnection with these inductances leads to additional insertion loss in the passband. In addition, and this is even more serious, the inductances reduce the reflectivity of the duplexers considerably in the case of out-of-band frequencies further away from the passband. In most cases, this does not cause any interference as long as the duplexer is operated in single mode, that is to say not in CA mode. However, the reduced reflectivity becomes a considerable problem if the duplexer is operated as part of a quadplexer since the lower reflectivity of the duplexer can then have a direct and negative effect on the insertion loss in the other duplexer at certain frequencies.
FIG. 2 shows a simulation of how the insertion loss IL of a filter or duplexer is additionally increased by an inductance which is connected in a parallel branch, as a function of the reflectivity REF at the corresponding antenna terminal. It is found that with a reflectivity from 0.8 and less a serious impairment of the duplexer characteristics must be expected. Even if the duplexers are matched perfectly, and have ideal, that is to say lossless matching elements for matching to the common antenna terminal, a reflectivity of 0.8 would even in this ideal case lead to a loss of approximately 0.45 dB in the passband as shown by curve K1. Each worsening of the matching would then lead to even higher reflectivity and correspondingly higher losses. A reflectivity of 0.6 more would lead to more than 1 dB increased insertion loss. The situation becomes worse if the antenna terminal has a poorer reflection coefficient (S22) which is shown by the other curves in the figure.
FIG. 3 shows by means of a simulation the reflection coefficient REF, plotted against the frequency FR, of a duplexer at the antenna terminal as a function of the quality factor Q of the inductance used which is plotted for values of Q=50 (bottom curve) to Q=300 (top curve). It is found that the quality factor also has a considerable influence on the reflectivity and thus on the insertion loss. The values shown in the figure are calculated for a duplexer which has in the first parallel branch an inductance in series with the parallel resonator, that is to say in the parallel branch which is located nearest to the antenna terminal in the interconnection of the branches. A further duplexer, the band of which is either below or above the passband shown, will have high losses due to the poor reflectivity. This effect cannot be eliminated even by an ideal coil or subsequent matching elements.
Apart from the disadvantage with respect to the increased reflectivity, the inductance also shows an advantage in that it improves the RX/TX insulation in the passband of the RX filter. Considering also the individual duplexer, the insertion loss is reduced only insignificantly by an inductance having a quality factor of 50.